Two sources familiar with the matter say that federal prosecutors will get testimony from the top adviser to Donald Trump after he was granted limited immunity from prosecution.
The immunity, a powerful tool that forces witnesses to testify on the promise that they will not be prosecuted for their statements or information derived from their statements, takes effect on 2 November.
Clarence Thomas was seen as a key player in efforts to challenge the election.
The justice department is interested in how the documents ended up at Mar-a-Lago and how Trump aides and lawyers responded to requests for their return, according to sources.
The status of the documents is important because if prosecutors can prove that those seized by the FBI in August were not declassified, it could strengthen a potential obstruction case.
Since the Mar-a-Lago search, Trump and his advisers have claimed multiple times that the seized documents were declassified, but no evidence has emerged to back up their claims.
The declassified documents may be relevant to the inquiry into Trump's retention of documents at the Florida property, as well as whether he wanted to impede or obstruct the investigation if his claims were false.
The justice department is expected to ask Patel about the circumstances behind the documents at Mar-a-Lago, as he is an adviser to Trump and a personal friend of the former president.
The justice department was considering using immunity on Wednesday. According to the Wall Street Journal, the immunity order was sent to the lawyers.
After he was summoned earlier this month to testify before a federal grand jury in Washington, the push to get his testimony intensified.
The sources said that even if the documents were not declassified, making false public statements would not be a crime.
If Trump made false statements about his campaign's ties to Russia to Congress or the FBI, they would be considered criminal.
The justice department applied for an order giving him limited immunity from prosecution after the chief US district court judge in Washington agreed that he had reason to assert the fifth.
In the last few weeks, Trump has doubled down on his claim that all of the documents in his possession were declassified, while his office has said that the records were automatically declassified.
According to Trump, presidents have the power to declassify documents by the power of thought.
If you are the president of the United States, you can declassify even if you don't like what you hear, as long as you think about it.
As prosecutors continue to gather evidence against Trump through his current and former aides, the justice department is willing to grant use immunity to him.
In high-profile cases, prosecutors only grant immunity to witnesses as a last resort, since it makes it more difficult to prosecute them in the future, and the move requires internal approval at the highest levels of the justice department.
One of the sources said that the justice department pressured Walt to sit for an additional interview to answer questions about how Trump ordered him to remove boxes from a storage room at Mar-a-Lago.
After prosecutors indicated they were skeptical of his initial account of moving documents from the storage room, he resisted having another interview with them.